Fig. 2A & B: AP and lateral views demonstrated a nondisplaced fracture of the posterior distal tibia just above the ankle and not included on the film. The most striking finding is the enlargement of the calf together with fat replacement of the muscles. This finding is typical for Duchenne pseudohypertrophic muscular dystrophy.
Discussion: Duchenne pseudohypertrophic muscular dystrophy is a genetic disease inherited as a X-linked recessive characteristic. The abnormal gene is on the short arm of the X-chromosome at the Xp21 locus. This gene which is very large in size accounts for 1% of the X-chromosome. Using "reverse genetics" it has proved possible to identify the cause of this type of muscular dystrophy [1]. DNA probes from the abnormal gene have been used to produce messenger RNA. The protein responsible for Duchenne dystrophy has been identified and called dystrophin. This protein is normally found close to the cell membrane and the sarcolemmal membrane and is involved in coupling the electrical activity with muscular contraction. Lack of dystrophin is responsible for Duchenne pseudohypertrophic muscular dystrophy and Becker muscular dystrophy, a much rarer and milder form of muscular dystrophy. A mechanism to insert dystrophin into the cell and reverse the disease process has not yet been discovered. Dystrophin is normally found in smooth muscle, cardiac muscle and brain as well as skeletal muscle.
Elevation of muscle enzymes (creatine kinase) may be present at birth in these boys but clinical symptoms do not usually begin until early childhood with muscular weakness and gait disturbance. Motor development is delayed with progressive weakness involving initially the pelvic girdle and neck flexor muscles followed by weakness in the shoulder girdle and proximal limb muscles. Flexion contractures and lumbar lordosis develop in childhood. Late in the course of the disease, plain films of the lower extremity may demonstrate widening of the fibular head with a relatively small fibular shaft compared to the tibia (Kaufman's fibular sign).
Fatty replacement in the calf muscles particularly the soleus is responsible for pseudohypertrophic appearance and the plain film findings shown in this case. By early adolescence, most boys are wheelchair bound and then commonly develop scoliosis. Cardiomyopathy with congestive cardiac failure is apparent later in the disease. A form of dilated cardiomyopathy has been described also in men with a similar genetic abnormality but without the other features of muscular dystrophy[2]. Gastrointestinal paresis from smooth muscle involvement is described with intestinal pseudo-obstruction and rarely acute gastric dilatation[3]. This may be life threatening. The functional impairment can be diagnosed and monitored with radionuclide gastric emptying studies. Death from respiratory failure, pneumonia or cardiomyopathy occurs in early adulthood.
Imaging does not usually play a part in the initial diagnosis of muscular dystrophies in childhood. Involvement of muscle groups and progression of disease can be well monitored by MRI [4] and CT [5] Affected muscles may also show uptake on gallium scans and ultrasound can also be helpful for demonstrating thickening of the perimysial septa and following non-invasively muscle involvement [6]
References:
1. Zasloff M. Molecular genetics. Chapter In: Nelson Textbook of Pediatrics, Behrman
RE, Kliegman RM, Nelson WE, Vaughan III VC, Eds. Philadelphia: W.B. Saunders
Company, 1992:263-270.
2. Towbin JA, Hejtmancik JF, Brink P, et al. X-linked dilated cardiomyopathy: Molecular
genetic evidence of linkage to the Duchenne muscular dystrophy (dystrophin) gene at
Xp21 locus. Circulation 1993; 87:1854-65.
3. Barohn RJ, Levine EJ, Olson JO, et al. Gastric hypomotility in Duchenne's muscular
dystrophy. N Eng J Med 1988; 319:15-18.
4. Schreiber A, Smith WL, Ionasescu V, et al. Magnetic resonance imaging of children
with Duchenne muscular dystrophy. Pediatr Radiol 1987; 17:495-497.
5. O'Doherty DS, Schellinger D, Raptopoulos V. Computed tomographic patterns of
pseudohypertrophic muscular dystrophy: preliminary results. J Comp Assist Tomogr
1977;1: 482-486.
6. Dock W, Happak W, Grabenwoger, et al. Neuromuscular diseases: evaluation with
high-frequency sonography. Radiology 1990; 177:825-828.
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